Molecular biologists led by Leonid Pobezinsky and his wife and research collaborator Elena Pobezinskaya at the University of Massachusetts Amherst have published results that for the first time show how a microRNA molecule known as Lethal-7 (let-7) serves as a molecular control hub to direct the function of cytotoxic T lymphocytes by putting the brakes on their cell-killing activities.

Further, the researchers found that when let-7 levels are low or absent, the body's T cells can potentially turn into "super killers," Pobezinsky says. The discovery is a significant advance in the quest to enlist the body's own immune defenses, a technique known as adaptive immunotherapy. It may more effectively attack disease-causing agents such as invasive cancer and chronic infections like HIV, he adds. Details appear now in the open access journal eLife.

As Pobezinsky explains, "We get cancer because CD8 T-cells are not always efficient and cancer can overcome them. Our lab looks at the molecular mechanisms that regulate the cytotoxic efficiency of T cells." He and colleagues report finding the control mechanism in a tiny strand of microRNA, only 20-30 nucleotides long, that determines how effective T cells will be in attacking disease.

"This was very interesting because when microRNAs were discovered over 20 years ago, people thought it was a product of RNA degradation. They were considered used fragments, like dust. They are so tiny, nobody paid attention to them," he notes. "But since then, people have gradually been discovering what they really do. Our work is continuing that."

The research group led by the Pobezinskys includes his Ph.D. student and first author Alexandria Wells, UMass Amherst molecular biologist Michele Markstein, who provided the computational analysis to identify let-7 targets and how let-7 regulates the genome, and UMass Medical School immunologist Raymond Welsh, a specialist in cytotoxic CD8 T cells who provided a viral model for testing differentiation in the presence of virus.

Pobezinsky points out that normally RNA codes proteins, but microRNA do not.

Instead, these tiny RNA snips found in humans and mammals have regulatory activity on the whole genome. Further, he notes, "The specific microRNA known as Lethal 7 or let-7 is a very ancient RNA that existed in the first eukaryotes and has been conserved through evolution. Humans and animals have multiple genes that code for it instead of the usual one; only the most important genes are duplicated during evolution."

This series of experiments was sparked by the observation that T-cells produce a lot of let-7 molecules, and when T cells are in their inactive state with no pathogen present, "our T cells are full of these let-7 cells," Pobezinsky says. "But the moment they see an antigen, suddenly the let-7s are gone. So the question is what do they regulate and why do they need to disappear?"

The researchers hypothesized that in the presence of microRNA let-7 molecules, T cells are quiet, which for the organism is a safe condition with the immune system inactive. But when a threat is sensed, let-7 molecules disappear, which allows T cells to become functionally cytotoxic and able to clear pathogens, including tumor cells.

Pobezinsky says, "Our hypothesis turns out to be correct, in fact the microRNAs work as a brake on the cytotoxic T cells when there is no antigen present, so when we are healthy, they rest. As soon as they are gone, T cells initiate differentiation into cytotoxic T lymphocytes." To kill invaders, T cells inject a toxic molecules or granzymes into a cancer or virally infected cell that initiates its apoptosis, or programmed cell death.

In experiments with three groups of mice: wild type controls, mice genetically modified to have no let-7 and another group engineered to have a super abundance of let-7, the researchers found that the complete absence of let-7 yielded the strongest differentiation of T cells to killer status. "If you keep let-7, T cells cannot become cytotoxic even in the presence of tumor or virus," Pobezinsky says. "If you have none or almost none, function is enhanced. Nobody knew this before."

"We also figured out the molecular pathway using transcription factors that regulate the T cell differentiation and confirmed that let-7 microRNA is the most critical control," he adds. The researchers now hope that this might lead to the ability to modulate immune responses that are regulated by CD8 cells, and they are testing it on mouse tumor models to try to enhance immune response against tumors using this technique.

"We'd like to develop a way to suppress or enhance immune response," Pobezinsky says. "We might be able to combine this with adaptive immunotherapy to enhance immune function, so we would use a person's own T cells, treat them in vitro, then to put back super killer T cells to boost their immune response. It's very promising, I feel it's a real possibility to go from this fundamental research and have an immediate application. For us it is very, very satisfying to do something for society."

Cancer immunotherapies that block two different checkpoints on T cells launch immune attacks on cancer by expanding distinct types of T cell that infiltrate tumors, researchers from The University of Texas MD Anderson Cancer Center report in the journal Cell.

"The mechanisms these two therapies use mostly do not overlap, which provides a reason why combining them works better than either alone," said Jim Allison, Ph.D., chair and professor of Immunology at MD Anderson.

"The clinical successes of checkpoint blockade have gotten out ahead of our understanding of how these drugs work," Allison said. "In some ways, that's a good problem to have, but we need greater understanding of the basic science behind these drugs to use them more effectively for patients."

Uncovering the cellular mechanisms used by each type of checkpoint inhibitor opens the door to more precise understanding of how to use the drugs separately and in combination with each other and other types of therapy, Allison said.

When they analyzed immune infiltrates, lead author Spencer Wei, Ph.D., a postdoctoral fellow in Allison's lab, and colleagues found:

Anti-CTLA-4 treatment expands the presence of CD4 effector T cells that are positive for ICOS, an immune-stimulating protein, and that these cells were strongly associated with smaller tumors in the mice. Both anti-PD-1 and anti-CTLA-4 treatment greatly expand the presence of CD8 T cells, the most powerful killers in the T cell family, and this expansion was associated with smaller tumors in the mice. These PD-1 positive CD8 T cells had what scientists call an exhausted-like phenotype. They have markers of inactivity, including the presence of other immune checkpoints, but are not necessarily known to be inactive and likely still have significant functional activity.

"These cells are boosted by anti-PD-1, but they keep their exhausted phenotype, which suggests they'll shut down when the PD-1 antibody is withdrawn," Allison said. This supports the need to give the drug for long periods, and current anti-PD-1 regimens provide a year or two of treatment.

The team ran these experiments in a mouse model of melanoma known to be highly immunogenic, so vulnerable to immunotherapy, and one model that is poorly immunogenic -- "more like prostate cancer than melanoma," Allison said.

The two drugs worked by expanding the same T cell infiltrates in both tumor types, so their mechanisms appear to be independent of tumor characteristics, but tumor characteristics are likely to affect the magnitude of responses.

Subsequent analyses of surgically removed human melanoma tumors showed that anti-CTLA-4 and anti-PD-1 each expanded T cell populations analogous to those found in the mouse models.

Additional research is needed to confirm and further understand these findings in larger studies, the researchers note.

Allison invented immune checkpoint blockade as a cancer therapy with his research leading to development of ipilimumab (Yervoy), and an approach of treating the immune system, rather than the tumor directly. By blocking CTLA-4, a protein on T cells that shuts down immune responses, ipilimumab unleashed the adaptive immune system to attack cancer.

Subsequently, other researchers developed inhibitors that block PD-1, a separate brake on T cells, or its main activating ligand, PD-L1, found on tumors and on normal cells.

Ipilimumab has been approved by the U.S. Food and Drug Administration alone or in combination against metastatic or inoperable melanoma. Two PD-1 inhibitors have been approved for melanoma, lung, bladder, kidney, head and neck cancers and non-Hodgkin lymphoma. Several PD-L1 inhibitors have also been approved for use in multiple tumor types.

CTLA-4 blockade is thought to act at the initiation of immune response, while PD1 is used by tumors to shut down an immune response that is under way by using PD-L1 to turn T cells off. About 15 to 25 percent of patients respond to these drugs, and researchers are seeking biomarkers to guide treatment and exploring new combinations to improve and expand responses.

Allison and Sharma are co-directors of the Parker Institute for Cancer Immunotherapy at MD Anderson. Allison holds the Lilian H. Smith Distinguished Chair in Immunology. Wei is an MD Anderson Odyssey postdoctoral fellow.

Cogdill is a graduate student in the MD Anderson UTHealth Graduate School of Biomedical Sciences.

Allison in the past has received royalties from Bristol-Myers Squibb for ipilimumab but no longer receives such payments.

This research was funded by support and grants from MD Anderson's Cancer Center Support Grant from the National Cancer Institute of the National Institutes of Health (P30CA16672), the Cancer Prevention and Research Institute of Texas, Memorial Sloan Kettering Cancer Center Cancer Center Support Grant (P30CA008748) and NIH grants (DP1-HD084071, RO1CA164729 and RO1CA163793).

Antibodies to the proteins PD-1 and PD-L1 have been shown to fight cancer by unleashing the body's T cells, a type of immune cell. Now, researchers at the Stanford University School of Medicine have shown that the therapy also fights cancer in a completely different way, by prompting immune cells called macrophages to engulf and devour cancer cells.

The finding may have important implications for improving and expanding the use of this cancer treatment, the researchers said.

A study describing the work, which was done in mice, published online May 17 in Nature. The senior author is Irving Weissman, MD, professor of pathology and of developmental biology. The lead author is graduate student Sydney Gordon.

PD-1 is a cell receptor that plays an important role in protecting the body from an overactive immune system. T cells, which are immune cells that learn to detect and destroy damaged or diseased cells, can at times mistakenly attack healthy cells, producing autoimmune disorders like lupus or multiple sclerosis. PD-1 is what's called an "immune checkpoint," a protein receptor that tamps down highly active T cells so that they are less likely to attack healthy tissue.

How cancer hijacks PD-1

About 10 years ago, researchers discovered that cancer cells learn to use this immune safeguard for their own purposes. Tumor cells crank up the production of PD-L1 proteins, which are detected by the PD-1 receptor, inhibiting T cells from attacking the tumors. In effect, the proteins are a "don't kill me" signal to the immune system, the Stanford researchers said. Cancer patients are now being treated with antibodies that block the PD-1 receptor or latch onto its binding partner, PD-L1, to turn off this "don't kill me" signal and enable the T cells' attack.

"Using antibodies to PD-1 or PD-L1 is one of the major advances in cancer immunotherapy," said Weissman, who is also the Virginia and D.K. Ludwig Professor for Clinical Investigation in Cancer Research, director of the Stanford Institute for Stem Cell Biology and Regenerative Medicine and director of the Ludwig Center for Cancer Stem Cell Research and Medicine at Stanford. "While most investigators accept the idea that anti-PD-1 and PD-L1 antibodies work by taking the brakes off of the T-cell attack on cancer cells, we have shown that there is a second mechanism that is also involved."

What Weissman and his colleagues discovered is that PD-1 activation also inhibits the anti-cancer activity of other immune cells called macrophages. "Macrophages that infiltrate tumors are induced to create the PD-1 receptor on their surface, and when PD-1 or PD-L1 is blocked with antibodies, it prompts those macrophage cells to attack the cancer," Gordon said.

Similar to anti-CD47 antibody

This mechanism is similar to that of another antibody studied in the Weissman lab: the antibody that blocks the protein CD47. Weissman and his colleagues showed that using anti-CD47 antibodies prompted macrophages to destroy cancer cells. The approach is now the subject of small clinical trials in human patients.

As it stands, it's unclear to what degree macrophages are responsible for the therapeutic success of the anti-PD-1 and anti-PD-L1 antibodies.

The practical implications of the discovery could be important, the researchers said. "This could lead to novel therapies that are aimed at promoting either the T-cell component of the attack on cancer or promoting the macrophage component," Gordon said.

Another implication is that antibodies to PD-1 or PD-L1 may be more potent and broadly effective than previously thought. "In order for T cells to attack cancer when you take the brakes off with antibodies, you need to start with a population of T cells that have learned to recognize specific cancer cells in the first place," Weissman said. "Macrophage cells are part of the innate immune system, which means they should be able to recognize every kind of cancer in every patient."

The University of Texas MD Anderson Cancer Center report in Science Translational Medicine.

Whole exome sequencing of tumor biopsies taken before, during and after treatment of 56 patients showed that outright loss of a variety of tumor-suppressing genes with influence on immune response leads to resistance of treatment with both CTLA4 and PD1 inhibitors.

The team's research focuses on why these treatments help 20-30 percent of patients -- with some complete responses that last for years -- but don't work for others. Their findings indicate that analyzing loss of blocks of the genome could provide a new predictive indicator.

"Is there a trivial or simple (genomic) explanation? There doesn't seem to be one," said co-senior author Andrew Futreal, Ph.D., professor and chair of Genomic Medicine and co-leader of MD Anderson's Moon Shots Program™. "There's no obvious correlation between mutations in cancer genes or other genes and immune response in these patients."

"We found a higher burden of copy number loss correlated to response to immune checkpoint blockade and to lower immune scores, a measure of immune activation in the tumor's microenvironment," said Roh, a graduate student in the University of Texas MD Anderson UTHealth Graduate School of Biomedical Sciences. "We also found copy loss has an effect that is independent of mutational load in the tumors."

Mutational load + copy loss tells a story

Melanoma tumors with larger volumes of genetic alterations, called mutational load, provide more targets for the immune system to detect and are more susceptible to checkpoint blockade, although that measure is not conclusive alone. "Combining mutational load and copy number loss could improve prediction of patient response," Wargo said. When the team stratified patients in another data set of patients by whether they had high or low copy loss or high or low mutational load, they found that 11 of 26 patients with high mutational load and low copy loss had a clinical benefit, while only 4 or 26 with low mutational load and high copy loss benefited from treatment.

In the trial, patients were treated first with the immune checkpoint inhibitor ipilimumab, which blocks a brake called CTLA4 on T cells, the immune system's specialized warriors, freeing them to attack.

Patients whose melanoma did not react then went on to anti-PD1 treatment (nivolumab), which blocks a second checkpoint on T cells. Biopsies were taken, when feasible, before, during and after treatment for molecular analysis to understand response and resistance.

To better understand the mechanisms at work, the team analyzed tumor genomes for recurrent copy loss among 9 tumor biopsies from patients who did not respond to either drug and had high burden of copy number loss. They found repeated loss of blocks of chromosomes 6, 10 and 11, which harbor 13 known tumor-suppressing genes.

Analysis of a second cohort of patients confirmed the findings, with no recurrent tumor-suppressor loss found among any of the patients who had a clinical benefit or long-term survival after treatment.

Ipilimumab sometimes wins when it fails

The researchers also found a hint that treatment with ipilimumab, even if it fails, might prime the patient's immune system for successful anti-PD1 treatment.

The team analyzed the genetic variability of a region of the T cell receptors, a feature of T cells that allows them to identify, attack and remember an antigen target found on an abnormal cell or an invading microbe. They looked for evidence of T cell "clonality," an indicator of active T cell response.

Among eight patients with longitudinal samples taken before treatment with both checkpoint types, all three who responded to anti-PD1 therapy had shown signs of T cell activation after anti-CTLA treatment. Only one of the five non-responders had similar indicators of T cell clonality.

"That's evidence that anti-CTLA4 in some cases primes T cells for the next step, anti-PD1 immunotherapy. It's well known that if you don't have T cells in the tumor, anti-PD1 won't do anything, it doesn't bring T cells into the tumor," Futreal says.

Overall, they found that T cell clonality predicts response to PD1 blockade but not to CTLA-4 blockade."Developing an assay to predict response will take an integrated analysis, thinking about genomic signatures and pathways, to understand the patient when you start therapy and what happens as they begin to receive therapy," Wargo said. "Changes from pretreatment to on-therapy activity will be important as well."

APOLLO tracks response over time

The Science Translational Medicine paper is the third set of findings either published or presented at scientific meetings by the team, which is led by Futreal and Wargo, who also is co-leader of the Melanoma Moon Shot™.

Immune-monitoring analysis showed that presence of immune infiltrates in a tumor after anti-PD1 treatment begins is a strong predictor of success. They also presented evidence that the diversity and composition of a patient's gut bacteria also affects response to anti-PD1 therapy.

The serial biopsy approach is a hallmark of the Adaptive Patient-Oriented Longitudinal Learning and Optimization™ (APOLLO) platform of the Moon Shots Program™, co-led by Futreal that systematically gathers samples and data to understand tumor response and resistance to treatment over time.

The Moon Shots Program™ is designed to reduce cancer deaths by accelerating development of therapies, prevention and early detection from scientific discoveries.

Whole exome sequencing of tumor biopsies taken before, during and after treatment of 56 patients showed that outright loss of a variety of tumor-suppressing genes with influence on immune response leads to resistance of treatment with both CTLA4 and PD1 inhibitors.

The team's research focuses on why these treatments help 20-30 percent of patients -- with some complete responses that last for years -- but don't work for others. Their findings indicate that analyzing loss of blocks of the genome could provide a new predictive indicator.

"Is there a trivial or simple (genomic) explanation? There doesn't seem to be one," said co-senior author Andrew Futreal, Ph.D., professor and chair of Genomic Medicine and co-leader of MD Anderson's Moon Shots Program™. "There's no obvious correlation between mutations in cancer genes or other genes and immune response in these patients."

"We found a higher burden of copy number loss correlated to response to immune checkpoint blockade and to lower immune scores, a measure of immune activation in the tumor's microenvironment," said Roh, a graduate student in the University of Texas MD Anderson UTHealth Graduate School of Biomedical Sciences. "We also found copy loss has an effect that is independent of mutational load in the tumors."

Mutational load + copy loss tells a story

Melanoma tumors with larger volumes of genetic alterations, called mutational load, provide more targets for the immune system to detect and are more susceptible to checkpoint blockade, although that measure is not conclusive alone. "Combining mutational load and copy number loss could improve prediction of patient response," Wargo said. When the team stratified patients in another data set of patients by whether they had high or low copy loss or high or low mutational load, they found that 11 of 26 patients with high mutational load and low copy loss had a clinical benefit, while only 4 or 26 with low mutational load and high copy loss benefited from treatment.

In the trial, patients were treated first with the immune checkpoint inhibitor ipilimumab, which blocks a brake called CTLA4 on T cells, the immune system's specialized warriors, freeing them to attack.

Patients whose melanoma did not react then went on to anti-PD1 treatment (nivolumab), which blocks a second checkpoint on T cells. Biopsies were taken, when feasible, before, during and after treatment for molecular analysis to understand response and resistance.

To better understand the mechanisms at work, the team analyzed tumor genomes for recurrent copy loss among 9 tumor biopsies from patients who did not respond to either drug and had high burden of copy number loss. They found repeated loss of blocks of chromosomes 6, 10 and 11, which harbor 13 known tumor-suppressing genes.

Analysis of a second cohort of patients confirmed the findings, with no recurrent tumor-suppressor loss found among any of the patients who had a clinical benefit or long-term survival after treatment.

Ipilimumab sometimes wins when it fails

The researchers also found a hint that treatment with ipilimumab, even if it fails, might prime the patient's immune system for successful anti-PD1 treatment.

The team analyzed the genetic variability of a region of the T cell receptors, a feature of T cells that allows them to identify, attack and remember an antigen target found on an abnormal cell or an invading microbe. They looked for evidence of T cell "clonality," an indicator of active T cell response.

Among eight patients with longitudinal samples taken before treatment with both checkpoint types, all three who responded to anti-PD1 therapy had shown signs of T cell activation after anti-CTLA treatment. Only one of the five non-responders had similar indicators of T cell clonality.

"That's evidence that anti-CTLA4 in some cases primes T cells for the next step, anti-PD1 immunotherapy. It's well known that if you don't have T cells in the tumor, anti-PD1 won't do anything, it doesn't bring T cells into the tumor," Futreal says.

Overall, they found that T cell clonality predicts response to PD1 blockade but not to CTLA-4 blockade."Developing an assay to predict response will take an integrated analysis, thinking about genomic signatures and pathways, to understand the patient when you start therapy and what happens as they begin to receive therapy," Wargo said. "Changes from pretreatment to on-therapy activity will be important as well."

APOLLO tracks response over time

The Science Translational Medicine paper is the third set of findings either published or presented at scientific meetings by the team, which is led by Futreal and Wargo, who also is co-leader of the Melanoma Moon Shot™.

Immune-monitoring analysis showed that presence of immune infiltrates in a tumor after anti-PD1 treatment begins is a strong predictor of success. They also presented evidence that the diversity and composition of a patient's gut bacteria also affects response to anti-PD1 therapy.

The serial biopsy approach is a hallmark of the Adaptive Patient-Oriented Longitudinal Learning and Optimization™ (APOLLO) platform of the Moon Shots Program™, co-led by Futreal that systematically gathers samples and data to understand tumor response and resistance to treatment over time.

The Moon Shots Program™ is designed to reduce cancer deaths by accelerating development of therapies, prevention and early detection from scientific discoveries.

Results of an initial study of tumors from patients with lung cancer or head and neck cancer suggest that the widespread acquired resistance to immunotherapy drugs known as checkpoint inhibitors may be due to the elimination of certain genetic mutations needed to enable the immune system to recognize and attack malignant cells. The study, conducted by researchers on the cells of five of their patients treated at the Johns Hopkins Kimmel Cancer Center, is described online Dec. 28 in Cancer Discovery.

"Checkpoint inhibitors are one of the most exciting recent advances for cancers, but the mechanism by which most patients become resistant to these therapies has been a mystery," says Victor E. Velculescu, M.D., Ph.D., program leader in the Bloomberg~Kimmel Institute for Cancer Immunotherapy at Johns Hopkins and professor of oncology. Clinical trials with the drugs to date have shown that nearly half of patients with lung cancers eventually develop resistance to this class of drugs for reasons that have been unclear.

Checkpoint inhibitors, such as nivolumab and ipilimumab, approved by the FDA for use against lung cancer, metastatic melanoma, head and neck cancer, and Hodgkin lymphoma, help the immune system recognize cancer cells by revealing evidence of mutated proteins called neoantigens on the surface of cancer cells.

To investigate why checkpoint inhibitors so often stop working, Velculescu; Valsamo Anagnostou, M.D., Ph.D., instructor of oncology at the Johns Hopkins University School of Medicine; Kellie N. Smith, Ph.D., a cancer immunology research associate at the Johns Hopkins University School of Medicine; and their colleagues at the Bloomberg~Kimmel Institute for Cancer Immunotherapy studied tumors of four patients with non-small cell lung cancer and one patient with head and neck cancer who developed resistance to two different checkpoint inhibitors: a drug called nivolumab that uses an antibody called anti-PD-1, or nivolumab used alone or in combination with a second drug called ipilimumab, which uses an antibody called anti-CTLA4.

Using biopsies of the patients' tumors collected before the start of treatment and at the time patients developed resistance, the researchers performed large-scale genomic analyses to search for mutations specific to the cancer cells in all of each patient's 20,000 genes.

The scientists' genomic search narrowed in on genes that code for the production of antigens, which serve as a source of identification to the immune system. Cancer cells may contain mutations in genes that code for antigens, producing misshapen or otherwise altered antigens that are known to scientists as neoantigens. Such neoantigens are foreign to the immune system, and thus, the cancer cell is flagged for destruction, usually with the help of immunotherapy drugs.

The scientists found that after the patients developed resistance to immunotherapy, all of their tumors had shed between seven and 18 mutations in neoantigen-coding genes. By getting rid of those mutations, the tumor cells' neoantigens look less foreign to the immune system and may go unrecognized, say the scientists.

The researchers found that the tumors had lost these mutations by various means, including immune-mediated elimination of cancer cells containing these mutations, leaving behind cancer cells without the mutations, or by deleting large regions of their chromosomes in all cancer cells.

"In some instances," says Anagnostou, "we found that chromosomes in the cancer cells' nuclei were missing an entire arm containing these mutated genes."

To confirm that the missing mutations were important for generating an immune response, the researchers cultured a subset of the neoantigen protein fragments containing the tumor mutations with immune cells taken from three patients' blood samples. Between one and six of the eliminated neoantigens were shown to generate a specific immune cell response in each of the patients.

"Our findings offer evidence about how cancer cells evolve during immunotherapy," Velculescu says. "When the cancer cells shed these mutations, they discard the evidence that would normally lead them to be recognized by the body's protective immune cells."

Velculescu, Anagnostou, Smith and their colleagues say they plan to determine how broadly this phenomenon occurs in other cancer types and potentially use it to develop new ways to improve current cancer immunotherapies. For example, they say, the mutated neoantigens present in tumor cells before therapy could give clues as to which patients' tumors might develop resistance. The findings could also advance the development of new checkpoint inhibitors less likely to trigger resistance or personalized immunotherapy approaches.

Four checkpoint inhibitors are currently approved by the FDA -- nivolumab, pembrolizumab, atezolizumab and ipilimumab -- to treat melanoma, lymphoma, bladder, lung and head and neck cancers. The drugs cost more than $10,000 per month, an expense that has fueled efforts to improve their value and patient selection.

An existing drug known as a JAK inhibitor may help patients who don't respond to the so-called checkpoint inhibitor immunotherapy drugs overcome that resistance, suggests a new preclinical study published online in Cell today by Penn Medicine researchers. Importantly, the results demonstrate that shutting down the interferon pathway, shown here to be critical to a tumor's resistance to immunotherapy, with a JAK inhibitor may improve checkpoint inhibitor drugs and even bypass the need for combinations of these drugs, which often come with serious side effects.

Today's checkpoint inhibitor drugs target receptors such as PD1 and CTLA-4, which act as a type of "off switch" on a T cell to prevent it from attacking other cells. Inhibiting these pathways with one or a more of the drugs releases these "brakes" so the immune system can fight the disease. However, over half of patients on the drugs relapse or their cancer progresses.

"The proposed approach has some elegance to it -- rather than try to figure out all inhibitory pathways that the tumor has enabled, find a critical pathway that regulates many of the inhibitory signals and cripple that instead," said senior author Andy J. Minn, MD, PhD, an assistant professor of Radiation Oncology in the Perelman School of Medicine at the University of Pennsylvania. "Interferon signaling is like a critical node in a network. Disable it and a large part of that network collapses."

Using breast cancer and melanoma mouse models, Minn, first-author Joseph L. Benci, a graduate student in Penn's Cell and Molecular Biology Graduate Group, and their colleagues from the departments of Radiation Oncology, Abramson Family Cancer Research Institute and Penn's Parker Institute for Cancer Immunotherapy showed that prolonged interferon signaling in tumor cells increased resistance to checkpoint inhibitors through multiple inhibitory pathways, and that blocking this response resulted in improved survival and powerful tumor responses.

Authors on the paper also include Robert Vonderheide, MD, DPhil, the Hanna Wise Professor in Cancer Research, Amit Maity, MD, PhD, a professor of Radiation Oncology, and E. John Wherry, PhD, a professor of Microbiology and director of the Institute for Immunology at Penn.

Studies have shown that combining checkpoint inhibitors, ipilimumab and pembrolizumab, for instance, as well as adding radiation therapy, as described in a Penn paper from the same researchers in Nature in 2015, elicits promising tumor responses in patients. But many still do not respond because of additional unidentified "brakes."

Researchers modeled this unknown resistance in breast cancer and melanoma mouse models with various lab techniques, including the genetic tool CRISPR, and found that treating the mice with checkpoint inhibitors (against PD1 and/or CTLA4) with or without radiation, along with the JAK inhibitor ruxolitinib, effectively restored complete responses and long-term survival in mice with tumors that are normally highly resistant to therapy. Inhibiting this pathway could also bypass the need for multiple checkpoint inhibitors: one checkpoint inhibitor (anti-CTLA4) and the JAK inhibitor in the breast cancer mouse model resulted in a 100 percent complete response and survival.

JAK inhibitors, U.S. Food and Drug Administration-approved drugs to treat myelofibrosis and psoriasis, target the well-studied interferon pathway, typically considered to be immunostimulatory. However, the authors found that over time interferon signaling changes how cells respond epigenetically to molecular signals in the tumor, switching from stimulatory to suppressive, similar to what happens in a chronic viral infection. Thus, blocking it switched off the tumor's resistance in mice.

"To our surprise, blocking interferon driven resistance not only antagonizes multiple inhibitory pathways that hinders combination therapies in mice," Minn said, "but it may also provide a general strategy to the challenge of designing complex combination checkpoint blockade therapies that seek to address the well-known problem of resistance."

Downgrading the number of checkpoint inhibitors for therapy has its advantages, given the severe and sometimes life-threatening toxicities that come along with combination therapies, including autoimmune complications such as colitis and fatal myocarditis.

"There is a real translational implication here," Minn said. "Because the interferon signaling pathway is targetable pharmacologically, we could perhaps mimic what we did in mice using JAK inhibitors that already exist for other purposes."

The team is looking to begin a new clinical study in lung cancer patients based on their findings in the upcoming months. The researchers also identified two potential biomarkers, MX1 and IFIT1, that may help identify tumors in patients under the influence of this interferon suppression.

A Ludwig Cancer Research study shows that an experimental drug currently in clinical trials can reverse the effects of troublesome cells that prevent the body's immune system from attacking tumors. The researchers also establish that it is these suppressive cells that interfere with the efficacy of immune checkpoint inhibitors. This class of immunotherapies lifts the brakes that the body imposes on the immune system's T cells to unleash an attack on cancer cells.

"Though checkpoint inhibitors have durable effects when they work, not all patients respond to the treatment," says Taha Merghoub, an investigator at the Ludwig Memorial Sloan Kettering Collaborative Laboratory who led the study with Director Jedd Wolchok. "Part of the reason for this is that some tumors harbor tumor-associated myeloid cells, or TAMCs, that prevent T cells from attacking tumor cells."

In a study published online in Nature, Merghoub and his team used mouse models of cancer to show that the effects of TAMCs can be reversed by an appropriately targeted therapy.

To show that TAMCs were indeed involved in resistance to checkpoint blockade, the researchers used a specific growth stimulant to increase their number in melanoma tumors to create a suitable model for their studies. They found that this made the tumors less susceptible to checkpoint blockade.

"We were able to make a tumor that was not rich in immune suppressing myeloid cells into one that was," says Merghoub.

Having established a link between TAMCs and checkpoint inhibitor resistance, the researchers next set out to test the hypothesis that blocking immune suppressor cell activity would improve immunotherapy response. To do this, they used an experimental drug manufactured by Infinity Pharmaceuticals called IPI-549. The drug, which is available for clinical use, blocks a molecule in the suppressor cells called PI3 kinase-gamma. Blocking this molecule changes the balance of these immune suppressor cells in favor of more immune activation.

"We effectively reprogrammed the TAMCs, turning them from bad guys into good guys," Merghoub said.

IPI-549 dramatically improved responses to immune checkpoint blockade (ICB) therapy for tumors with high concentrations of TAMCs. When checkpoint inhibitors were administered to mice with suppressed tumors, only 20% of the animals underwent complete remission. When the same drugs were administered with IPI-549, that number jumped to 80%. IPI-549 provided no benefit to tumors lacking the suppressor cells.

Merghoub and his team also showed that tumors that were initially sensitive to checkpoint inhibitors were rendered unresponsive when their TAMC concentrations were boosted with growth stimulants.

Taken together, these results indicate that TAMCs promote resistance to checkpoint inhibitors and that IPI-549 can selectively block these cells, thereby overcoming their resistance.

Merghoub said the findings help pave the way for a precision medicine approach to immunotherapy that will allow cancer treatments to be tailored to a patient's particular tumor profile. "We can now potentially identify patients whose tumors possess immune suppressor cells and add a drug to their treatment regimen to specifically disarm them," he added.

IPI-549 is currently undergoing a Phase I trial in the United States to assess its safety when administered alone and in combination with the FDA-approved checkpoint inhibitor drug nivolumab (Opdivo®).

One of the major obstacles with treating cancer is that tumors can conscript the body's immune cells and make them work for them. Researchers at EPFL have now found a way to reclaim the corrupted immune cells, turn them into signals for the immune system to attack the tumor, and even prevent metastasis.

Macrophages are cells of the immune system that protect the host from invading pathogens. But in cancer, macrophages can be "hijacked" by tumors, and made to support their malignant growth and spread. This is a drawback for a major cancer treatment, immunotherapy, which turns the body's immune system against the tumor. EPFL scientists, working with colleagues at the Roche Innovation Centers in Munich and Basel, have now identified a molecular "switch" that can convert the "hijacked" macrophages into cells that can stimulate the immune system to fight the growth and spread of cancer. The work is published in Nature Cell Biology.

"Traitor" macrophages

Along with attacking foreign pathogens like bacteria, macrophages also help the body's organs develop and its wounds heal. Their own behavior is fine-tuned by small molecules that they produce, called microRNAs.

When a tumor begins to develop, macrophages attempt to block its growth. But often tumors hijack them and convert them into what are known as "tumor-associated macrophages," or TAMs for short.

Now corrupted, TAMs use their microRNAs to shield the tumor from the patient's immune system, helping it grow and metastasize. This phenomenon is common across many tumor types. It is one of the major obstacles in treating cancer, and often leads to a poor prognosis for the patient.

Reprogramming macrophages

Michele De Palma's team at EPFL found how to reclaim TAMs. The researchers genetically modified TAMs to remove their ability to produce microRNAs. As a result, the TAMs were reprogrammed dramatically. Instead of protecting the tumor, the TAMs now signaled the presence of the tumor to the immune system, triggering attacks against it -- and did so very efficiently.

Using a bioinformatics approach, the researchers found that the most likely culprit was a small family of microRNAs, called Let-7. This offers a more specific target: blocking Let-7 microRNAs may help instruct the TAMs to stimulate anti-tumor immunity.

Interestingly, the researchers observed that reprogramming TAMs also stops cancer cells from leaving the primary tumor. This could mean that the approach can also prevent tumor metastasis, the most threatening aspect of cancer. Moreover, the researchers found that the re-educated TAMs could enhance the anti-tumoral efficacy of certain cancer immunotherapies, some of which are already approved for patients.

However, more work is needed to translate all these findings to actual therapies, especially since there is currently no way to block the Let-7 microRNAs selectively in TAMs. But De Palma's lab is now working with bioengineers at EPFL to design drugs that can target the Let-7 microRNAs specifically in the TAMs.

Therapeutic opportunities

Some of the most promising cancer treatments are immunotherapies, which are based on provoking or enhancing the patient's immune response against their tumor. "The most exciting finding was that TAM reprogramming greatly improved the efficacy of immunotherapy," says Michele De Palma. "Our results in experimental models of cancer suggest a new therapeutic strategy based on inhibiting the microRNA machinery -- or the Let-7 microRNAs -- specifically in the TAMs, which may unleash the power of mainstream immunotherapies, such as immune checkpoint inhibitors."

Cancer researchers already know of some oncogenes and other factors that promote the development of colon cancers, but they don't yet have the full picture of how these cancers originate and spread. Now researchers from the Perelman School of Medicine at the University of Pennsylvania have illuminated another powerful factor in this process.

"This work reveals and unravels an additional pathway for the origin of colon cancer," said senior author Anil K. Rustgi, MD, the T. Grier Miller Professor of Medicine and chief of the Gastroenterology division.

Explorations of this pathway could lead to new ways of categorizing and treating colon cancers, which, together with less common rectal cancers, kill about 5,000 Americans every year.

The research, published this week in PLoS Genetics, follows a 2013 study in Genes and Development from Rustgi's group, which found that a protein called LIN28B promotes cancerous growth in intestinal cells by suppressing the Let-7 family of molecules.

LIN28B has attracted keen interest among biologists in recent years. The protein's suppression of Let-7 molecules normally helps keep embryonic stem cells in their stem-like state, not only in humans and other mammals but in evolutionarily distant species too. When Let-7 molecules are allowed to work, cells tend to move out of the stem-like state and mature into specific cell types, with much less capacity for uninhibited growth.

This ancient interaction between LIN28B and Let-7 is clearly important for the normal development of animals to maturity and for other growth-related processes such as tissue regeneration after injury. But as Rustgi and other scientists have been finding, LIN28B's suppression of Let-7 is also abnormally switched on in many cancers.

In the new study, Rustgi's team, including first author Blair B. Madison, PhD, who at the time was a postdoctoral fellow in the Rustgi laboratory and is now an assistant professor of Medicine at Washington University, looked downstream of the LIN28B/Let-7 interaction, to determine how Let-7 molecules normally keep intestinal cells from turning cancerous.

Let-7 molecules are not proteins. They are short stretches of RNA (microRNAs, or miRNAs) that work within cells to regulate the expression of various genes. To understand better what Let-7 miRNAs normally do to prevent cancer, Rustgi's team created transgenic mice that produce no Let-7 miRNAs in the intestinal lining.

The researchers observed that adenomas adenomatous polyps, as well as adenocarcinomas resembling typical human colon tumors, sprouted in the intestines of all these no-Let-7 mice by mid-adulthood, increasing their mortality compared to normal mice. Analyses of the tumors, and of derived "tumoroid" three-dimensional cell clusters cultured in the lab dish, pointed to a protein called Hmga2 as a major factor in the tumors' development.

Hmga2 is normally produced during the fast-growth period of fetal life and is thereafter suppressed by Let-7 miRNAs. Rustgi's team observed that in the intestinal lining of the no-Let-7 mice, as well as in tumors and derived tumoroids, Hmga2's gene was expressed at unusually high levels. Using antibodies to mark Hmga2 proteins, they found it to be particularly abundant in tumors that had begun to spread beyond the intestinal lining.

The researchers also found that experimentally lowering Hmga2's production, introduced by another line of transgenic mice, significantly suppressed tumors induced by Lin28b, and suppression of Let-7. What's more, experimentally lowering Hmga2 production in cultures of intestinal tissue from such mice significantly reduced the cells' tendency to proliferate, whereas increasing Hmga2 levels boosted that proliferation.

Analyses of gene expression in the tumors showed a strong relationship between the elevated expression of Hmga2 and the elevated expression of genes considered classic markers of stem cells. That observation adds to findings in recent years that many cancers, including colon cancers, may be driven in part by cancer cells that are in a stem-like state--which may enable them not only to proliferate more easily, but also to better withstand therapies.

Clearly, other factors were also at work in spurring the development of tumors in the Let-7-suppressed mice. Indeed, the researchers found evidence in the tumors of the overactivation of the Wnt signaling pathway, a known promoter of colon cancer--which in these cases may have become spontaneously switched on in some cells. "We suspect that that's the main dysregulation that occurs after Let-7 suppression to boost tumor progression," said Rustgi.

To check the relevance of these mouse results to humans, Rustgi's group examined several hundred human colorectal cancer samples, and found, among other things, lower-than-normal expression of Let-7 miRNAs, and higher-than-normal expression of HMGA2 (the human version of the mouse Hmga2 protein) as well as stem cell markers. In these human cancer samples, HMGA2 expression was also associated with a more advanced stage of tumor growth and reduced survival.

The findings point to a surge in HMGA2 as one of the key factors that promotes colon cancer in the many cases where Let-7 levels are suppressed. HMGA2 is already being considered as a target for new treatments for other cancer types, and this study suggests that targeting HMGA2--perhaps in concert with Wnt signaling factors--may make a difference in colon cancer too.

"We think that there's an axis of cancer promotion here, from LIN28B to Let-7 to the targets of Let-7, including HMGA2, and if one could disrupt the latter with therapeutics, that might help alleviate colon cancer progression and maybe metastasis as well," said Rustgi.

HMGA2 levels may also have a prognostic value, since in this study high HMGA2 levels correlated with more advanced and invasive tumors and a poorer outcome. "We might consider a different therapeutic approach for such patients," Rustgi noted.

(June 8, 2009) — A protein abundant in embryonic stem cells is now shown to be important in cancer, and offers a possible new target for drug development, report researchers from the Stem Cell Program at Children's Hospital Boston.

Last year, George Daley, MD, PhD, and graduate student Srinivas Viswanathan, in collaboration with Richard Gregory, PhD, also of the Stem Cell Program at Children's, showed that the protein LIN28 regulates an important group of tumor-suppressing microRNAs known as let-7. Increasing LIN28 production in a cell prevented let-7 from maturing, making the cell more immature and stem-like. Since these qualities also make a cell more cancerous, and because low levels of mature let-7 have been associated with breast and lung cancer, the discovery suggested that LIN28 might be oncogenic.

Now, publishing Advance Online in Nature Genetics on May 31, Daley, Viswanathan and colleagues show directly that LIN28 can transform cells to a cancerous state, and that it is abundant in a variety of advanced human cancers, particularly liver cancer, ovarian cancer, chronic myeloid leukemia, germ cell tumors and Wilm's tumor (a childhood kidney cancer). They believe that overall, LIN28 and a related protein, LIN28B, may be involved in some 15 percent of human cancers. By blocking or suppressing LIN28, it might be possible to revive the let-7 family's natural tumor-suppressing action.

"Linking this protein to advanced cancer is a very exciting new result," says Daley, Director of Stem Cell Transplantation at Children's, and also affiliated with Children's Division of Hematology/Oncology, the Dana-Farber Cancer Institute and the Harvard Stem Cell Institute. "It gives us a new target to attack, especially in the most resistant and hard-to-treat cases."

"Lier cette protéine au cancer est très excitant et nous donne une nouvelle cible à attaquer dans les cancers résistants et difficille à traiter."LIN28, which is abundant in embryonic stem cells and prevents them from differentiating into specific cell types, was originally discovered to influence embryonic development in worms some 25 years ago. Development, stem cell generation and carcinogenesis are known to be closely related, but until last year's study connected LIN28 to let-7, it hadn't been clear how.

"LIN28 is a fascinating protein that acts both in stem cells and cancers, and is teaching us that cancer is often a disease of stem cells," says Daley.

Viswanathan, Daley and colleagues are busily searching for ways to inhibit LIN28, which could provide promising new drugs for advanced cancer.

The study was funded by the National Institutes of Health, the NIH Director's Pioneer Award, Burroughs Wellcome Fund, the Leukemia and Lymphoma Society and the Howard Hughes Medical Institute.

Cancer afflicts 1.5 million people a year in the United States alone, and lung cancer is the most common and deadly form of cancer worldwide. This study indicates a direct role for a miRNA in cancer progression and introduces a new paradigm of using miRNAs as effective therapeutic agents to treat human cancer.

"We believe this is the first report of a miRNA being used to a beneficial effect on any cancer, let alone lung cancers, the deadliest of all cancers worldwide," said senior author Frank Slack, associate professor of molecular, cellular and developmental biology at Yale.

Slack's research group initially discovered the let-7 miRNA in C. elegans, a tiny worm used as a model system for studying how organisms develop, grow and age. They went on to show that in humans, let-7 negatively regulates a well-known determinant of human lung cancers, the RAS oncogene.

Let-7 régule à la baisse l'oncogène Ras sur le cancer du poumon.

In collaboration with scientists at Asuragen, the Slack lab has studied the tumor suppressor activity of this small RNA. Their work revealed that let-7 is commonly present at substantially reduced levels in lung tumors -- and that reduced levels of let-7 likely contribute to the development of the tumors. These discoveries focused public attention and research efforts to understand the potential use of naturally occurring microRNAs like let-7 to combat cancer.

This new work demonstrates that let-7 inhibits the growth of lung cancer cells in culture and in lung tumors in mice. They also showed that let-7 can be applied as an intranasal drug to reduce tumor formation in a RAS mouse model lung cancer.

"We believe that our studies provide the first direct evidence in mammals, that let-7 functions as a tumor suppressor gene," said Slack. "Because multiple cell lines and mouse models of lung cancer were used, it appears that therapeutic application of let-7 may provide benefits to a broad group of lung cancer patients."

"This has been a very productive industry-academic collaboration between Yale and Asuragen scientists" commented Matt Winkler CEO of Asuragen. "This work provides further evidence of the importance of miRNAs in the development of cancer and provides additional support for miRNA replacement therapy as an important component of effective cancer treatment regimens of the future."

Les miARNs seront une composante importante des thérapies futures.

Other authors on the paper were Aurora Esquela-Kerscher, Phong Trang and Joanne Weidhaas at Yale; Jason Wiggins, Lubna Patrawala, David Brown and Andreas Bader at Asuragen, Inc.; Angie Cheng and Lance Ford at Ambion, Inc. The work was funded by a grant from the State of Connecticut Department of Public Health and fellowships from the National Institutes of Health.

"If certain forms of breast cancer do indeed have their origin in wayward stem cells, as we believe to be the case, then it is critical to find ways to selectively attack that tumor-initiating population," said Gregory Hannon, Ph.D., CSHL professor and Howard Hughes Medical Institute Investigator. Hannon also is head of a lab focusing on small-RNA research at CSHL and corresponding author of a paper reporting the new research, published in Genes and Development.

"We have shown that a microRNA called let-7, whose expression has previously been associated with tumor suppression, can be delivered to a sample of breast-tissue cells, where it can help us to distinguish stem-like tumor-initiating cells from other, more fully developed cells in the sample. Even more exciting, we found that by expressing let-7 in the sample, we were able to attack and essentially eliminate, very specifically, just that subpopulation of potentially dangerous progenitor cells."

The study was done in collaboration with Senthil Muthuswamy Ph.D., an expert in breast cancer research who heads a CSHL lab focusing on understanding the changes in the biology of breast epithelial cells during the initiation and progression of cancer. Dr. Muthuswamy emphasized that a key ingredient that made this study successful is the use of a mouse breast-derived model cell system called COMMA-1D that not only includes differentiated cells but also stem-like progenitors, in varying stages of maturity, or differentiation.

Unexpected Impact of Conventional Chemotherapy

No therapies currently exist that target stem-like tumor-initiating cells, whose existence in diverse tissues including breast, lung, brain and colon, as well as in the blood, has been demonstrated in a line of research stretching back to 2001. In that year, John E. Dick of the University of Toronto identified cancer stem cells in the blood of leukemia patients.

The cancer stem cell hypothesis is controversial, in part, because of the challenge it represents for current cancer therapy, which regards all tumor cells as potentially capable of spreading the disease, and which seeks to reduce tumor mass and destroy the maximum possible number of tumor cells. In the cancer stem cell hypothesis, reduction of tumor volume alone will not suffice if the stem cells which originally gave rise to the cancer are not specifically targeted and destroyed.

The new Cold Spring Harbor Laboratory research not only suggests one possible way of accomplishing this therapeutic goal -- the Hannon lab is initiating a demonstration study in mice -- but it also demonstrated that one component of a chemotherapy cocktail currently used as first-line therapy against certain kinds of breast cancer has the potential to actually enrich the subpopulation of stem-like cells that serve as cancer progenitors.

"We found that administration of cyclophosphamide in our mouse cell sample had the effect of enriching for these cells," Hannon said, "which suggests that we need to look carefully at these therapies in model systems to see if the effects we see in cell culture are mirrored in real tumors -- and then, to gauge what effect that has on metastasis and relapse following therapy."

It has been known for some time that stem and progenitor cells possess unique defenses, as compared with mature, or differentiated cells, which, unlike their stem-like "mothers" do not have the capacity to renew themselves or to generate multiple cell-types. Stem cells, for instance, are thought to be able to "pump" toxins out of their cellular domain, much as do fully differentiated tumor cells that have developed resistance to chemotherapy.

(Dec. 14, 2007) — One of the biggest stories in cancer research over the past few years has been, unexpectedly, stem cells. Not embryonic stem cells, but tumor stem cells. These mutated cells, which live indefinitely and can seed new tumors, are now suspected of causing many, if not all, cancers. What is worse, these persistent cells are not killed by chemotherapy or other current treatments. Their survival might explain why tumors frequently recur or spread after treatment. Increasingly, researchers view the challenge of getting rid of these bad seeds as the key to treating cancer far more effectively. However, because they are extremely rare, even in large tumors, studying them has been difficult.

Now, researchers have devised a way to generate large numbers of human breast cancer stem cells in mice and have discovered a genetic switch that regulates critical properties of the cells. The regulator, which belongs to a class of molecules called microRNAs (microRNAs), pushes the stem cells to become more differentiated and less tumorigenic through its ability to switch off particular genes.

"People know that microRNAs are important regulators of cell differentiation, but nobody has shown that they regulate the critical properties of cancer stem cells, or any kind of stem cells," says Judy Lieberman, an investigator at the Immune Disease Institute and Harvard Medical School professor of pediatrics at Children's Hospital Boston. Lieberman and Erwei Song, a former postdoc in her lab now working as a breast cancer surgeon at Sun Yat-Sen University in Guangzhou, China, are the senior investigators on the work, which appears in the Dec. 14 issue of Cell.

By showing that microRNAs can rein in tumor stem cells, the work suggests a novel way to target these cells to treat cancer with therapeutic RNAs, a promising new class of medicine under development for many diseases.

In the study, Song and first author Fengyan Yu started working in China to isolate breast cancer stem cells from freshly removed tumors. Because cancer stem cells resist chemotherapy, the researchers predicted that breast tumors from women who had received such treatment before surgery might be enriched with stem-like cells, and their experiments confirmed this idea. In tumors from untreated women, less than 1 in 250 cells had the cell surface markers and growth characteristics of stem cells; in treated tumors, the number rose to 1 in 17.

The finding gave Song and Yu the idea of trying to generate larger quantities of tumor stem cells by growing human breast cancer cells in immunosuppressed mice dosed with a chemotherapeutic agent. After three months of such a regimen, nearly 75 percent of the cells in the retrieved tumors displayed the properties of stem cells: they had the expected cell surface markers, were highly tumorigenic and metastatic in mice, were relatively drug resistant, and could be induced to differentiate into multiple kinds of breast tissue cells.

With a ready supply of cancer stem cells, the researchers were able to measure levels of microRNAs, small gene regulators that are known to influence a gene's ability to create proteins important for cell growth and differentiation. They found that cancer stem cells contained low amounts of several microRNAs compared to more mature tumor cells or stem cells that had differentiated in culture.

They zeroed in on a tumor-supressing microRNA called let-7. When the team activated let-7 in the stem cells, they lost their ability to self-renew and began to differentiate. The cells also became less able to form tumors in mice or to metastasize. Further studies showed that let-7 did this by switching off two cancer-related genes: the oncogene Ras, and HMG2A, which when switched off caused the cells to differentiate.

If this finding applies to other tumor types, let-7 may offer a unique opportunity to attack tumor stem cells using therapeutic RNA. Delivery of the let-7 RNA to tumors could potentially deplete stem cells by pushing them down the path of differentiation. Using small RNAs to treat disease is a topic Lieberman is quite familiar with--in 2003, her lab was the first to show therapeutic RNAs could work in an animal model of liver disease, and their work has since focused on devising methods for targeting RNAs to all kinds of cells. Yu, now a visiting student in the Lieberman lab, is looking at ways to deliver the let-7 RNA mimics to stem cells.

"One of the fundamental problems of all the therapies that we have is that they are not doing anything to these cells," Lieberman says. "If those turn out to be the cells that go on and form metastases and are resistant to chemotherapy and are responsible for relapses, and if your therapy isn't dealing with those cells and is, in fact, selecting for them, that is very worrisome."